Halogen atoms because of their large electron affinity are known to be reactive and play a major role in chemistry.
There has existed another class of molecules which possess electron affinities that are larger than those of any halogen atom. These molecules called superhalogens consist of a central atom, typically a metal, surrounded by halogen or oxygen atoms.
Electron affinity (EA), defined as the amount of energy necessary to remove an electron from an anion, plays a dominant role in reactivity. This is evidenced by halogen atoms whose electron affinities are larger than those of any other element in the periodic table. Halogen atoms such as F, Cl, and Br are among the most electronegative elements in the periodic table; they are very reactive and form salts. They possess rather large electron affinity (EA) which is defined as the energy gained when an extra electron is attached to these atoms. Cl has the highest electron affinity, namely, 3.6 eV.
There exists a class of molecules whose electron affinities are even larger than those of halogen atoms. Bartlett and collaborators were among the first to study such molecules nearly half a century ago. They showed that PtF6 can even oxidize O2 molecule [1] and Xe atom [2]. Several years later Gutsev and Boldyrev coined the word superhalogen to describe these highly electronegative species [3]. According to these authors, a superhalogen consists of a central metal atom surrounded by halogen atoms. When the number of these halogen atoms exceeds the maximal valence of the metal atom, the molecule possesses electron affinities that are much larger than that of the halogen atoms. In a series of subsequent theoretical studies, Boldyrev and collaborators showed that a large number of superhalogens, where the central metal atom is typically a sp element, are possible [4-8]. The first photoelectron spectrum of MX2−(M=Li, Na; X=Cl, Br, and I) was reported by Wang and co-workers [9]. Subsequent photoelectron spectroscopic studies and theoretical studies have further confirmed the existence of superhalogens in the gas-phase [10-14]. In a theoretical study, it was proposed that hydrogen atom can act as the “central atom” to form a superhalogen and it was shown that the vertical detachment energies of [HnFn+1]− can be extremely high [15]. Numerous other superhalogen anions, such as permanganate (MnO4−) [16], perchlorate (ClO4−), hexafluoride (AuF6− and PtF6−) [17-18], BO2− [19], MgxCly− [20] have also been reported. Because of their high EAs, superhalogens almost always exist as negative ions, usually as the anionic portions of salts. Since salts composed of superhalogens generally have a high oxidative property, there is considerable interest in the synthesis of species with high EAs.